The hazelnut butter can be made in advance and refrigerated but make sure it is close to room temperature before serving. If you don’t have any hazelnut oil, just replace with walnut or olive.
Preheat the grill. Scatter the hazelnuts on to a baking tray and place under the grill, toasting to a rich golden brown to create the roasted flavour. Allow to cool before beating into the butter with the oil and vinegar. Season with salt and pepper.
Grease a baking tray and place the fish on the tray, skin-side up. Brush the skin with the knob of butter and season